- Early Greeks
- Geocentric- the earth is the
center of everything.
- Planetai- the 7 wanderers, the
planets.
- Aristarchus- 312-230 BC, the
first person to propose the Heliocentric solar system.
- Erasthones- 276-194 BC, he
is the first to establish the size of the earth using angles of shadows
created by the noonday sun in two cities that were north and south of each
other (Syene and Alexandria.)
- Erasthones
calculates that the angles of the shadows are off by 7º or 1/50 of a
complete circle. He concludes the distance around the earth must be 50 times
the distance between these two cities.
- The distance from one city to the
other was 5000 stadia, approximately 788 km.
- 788km x 50 = 39,400 km
- Today the accepted distance is
40,075 km.
- Hipparchus (190 - 120 B.C.) he is best known for his
star catalogue. He also determines the length of a year within minutes.
- The Ptolomeic system 141 AD
- planets orbit the motionless earth
- their orbits were the pure and perfect circle
- Ptolomey could not explain retrograde motion
accurately.
- Copernicus- 1473-1543
- He was convinced earth was a planet like the others.
He though all the motions that could not be explained easily could have been
due to the fact that earth rotated.
- He placed the sun in the center of the solar system
- He had difficulty explaining and predicting where
planets would be at any time in their orbit because he too believed that the
planets had circular orbits.
- Tycho Brahe 1546-1601
- A Danish observer of the heavens. He makes detailed
observations of Mars and its orbit.
- He makes detailed observations using a telescope. He
does not believe the Copernican idea of the earth being just a planet; he
believes earth is the center.
- He hires an assistant to help him in 1600. He dies in
1601.
- Johannes Kepler (1571-1630)
- This is the assistant to Brahe.
He ushers in the new age of astronomy using Tycho’s data and a sound mind.
- Kepler derives three basic
laws of planetary motions.
- He develops his laws when he
realizes that Brahe’s data has flaws in its explanation.
- He concludes that the orbits of
planets are not circles but ellipses.
- Planets speed up as they approach
the sun.
-
After a long struggle, in
which he tried mightily to avoid his eventual conclusion, Kepler was forced
finally to the realization that the orbits of the planets were not the
circles demanded by Aristotle and assumed implicitly by Copernicus,
but were instead the "flattened circles" that we call ellipses.
* We
will discuss his other laws later.
Johannes Kepler
1571-1630
1.
First to correctly
explain planetary motion, thereby, becoming founder of celestial mechanics and
the first "natural laws" in the modern sense; being universal, verifiable, and
precise.
2.
First to explain the
process of vision by refraction within the eye;
3.
First to formulate
eyeglass designing for nearsightedness and farsightedness;
4.
First to explain the
use of both eyes for depth perception.
LAW 1:
The orbit of a planet/comet about the Sun is an ellipse with the Sun's center of
mass at one focus.
This is the equation for the eccentricity of an ellipse:
e = d
L
E= eccentricity d= distance between foci
L= length across the ellipse
A perfect circle has 0 eccentricity
A straight line has an eccentricity of 1.0
See the ESRT’s solar system data for
eccentricities of the planets.
This is what you will do on the
lab.
